Dementia is most often thought of as a memory disorder, an illness of the aging mind. In its initial stages, that's true  memory loss is an early hallmark of dementia. But experts in the field say dementia is more accurately defined as fatal brain failure: a terminal disease, like cancer, that physically kills patients, not simply a mental ailment that accompanies older age. (See the top 10 medical breakthroughs of 2008.)

That distinction is largely unfamiliar both to the general public and within the medical field, yet it is a crucial one when it comes to treatment decisions for end-stage dementia patients. Dr. Greg Sachs at the Indiana University Center for Aging Research says a lack of appreciation of the nature of dementia leads to misguided and often overly aggressive end-stage treatment. Five years ago, Sachs wrote a paper on such barriers to palliative end-of-life care for dementia patients, but he ran into difficulty explaining the findings to the editors of the major medical journal that published it. "The editors kept coming back to me and saying, 'But what do the patients die of? You don't die from dementia.' And I kept saying, 'Yes, they do. That's the whole point of the paper,' " says Sachs.

Now, a large, prospective study to be published in the Oct. 15 issue of the New England Journal of Medicine goes a long way toward identifying the true course of the slow-progressing disease, which affects some 5 million Americans  a number that is expected to triple by 2050. "This is the first large study to show what specialists have been arguing for years. Dementia is a terminal illness, and patients warrant palliative care," says Sachs, who wrote an editorial that appears in the same issue of the journal.

The new study followed 323 Boston-area nursing-home residents with advanced dementia for 18 months. These patients were unable to recognize family members, incontinent and unable to get around on their own. Researchers tracked the progression of their disease, complications and survival rates; they also recorded the treatments each patient received as well as their health-care proxies' understanding of advanced dementia and the patient's prognosis. Over the course of the study, 55% of the residents died, with nearly half of those deaths occurring within the first six months of the study. Patients' median survival span was 478 days, a figure comparable with that of terminal-cancer patients. Thirty-one residents suffered major health events, such as seizure, gastrointestinal bleeding, heart attack or stroke, but only in rare cases did those events lead to death. Only seven patients had a major event during the final three months of life. "Our main findings confirmed dementia has high mortality. People in the study didn't have other devastating things happen to them before they died," says the study's lead author, Dr. Susan Mitchell of the Harvard-affiliated Hebrew SeniorLife Institute for Aging Research. (Read "The Year in Medicine 2008: From A to Z.")

Dementia is not a single illness but a collection or consequence of many, including Parkinson's disease, vascular dementia and Alzheimer's disease (which accounts for some 70% of all dementia cases). In the advanced stages of dementia, it is often impossible to tell which disease the patient had at the outset, as the end result is the same, according to Mitchell's study: a syndrome of symptoms and complications  eating problems (86%), pneumonia (41%), difficulty breathing (46%), pain (39%) and fever (53%)  caused by brain failure. "Dementia ends up involving much more than just the brain," says Dr. Claudia Kawas, professor of neurology at the University of California, Irvine. "We forget the brain does everything for us  controls the heart, the lungs, the gastrointestinal tract, the metabolism."

When those systems fail, patients are often treated aggressively rather than with palliative care. More than 40% of residents who died over the course of the study were sent to the emergency room, hospitalized, tube-fed or given IV nutrition during the last three months of life. These interventions can themselves cause distress and pain while providing, at best, questionable benefit and minimal prolongation of life, experts say. Among the family members who directed these residents' care, however, those who believed that the resident had less than six months to live and understood the nature of advanced dementia were less likely to intervene aggressively than caregivers who lacked such understanding. "Clinicians, patients' families and nursing-home staff need to recognize and treat advanced dementia as a terminal illness requiring palliative care," wrote Sachs in his editorial, noting that patients need not be close to death to warrant pain-relieving treatment.

Experts say part of the reason it is so common to intervene in dementia cases is that the patient, by definition, cannot make medical decisions autonomously, leaving a relative or friend to serve as their health-care proxy. "Family members are much less likely to forgo treatments or let go. An 80-year-old patient will tell you, 'I have lived a good, long life. I have no regrets.' But talk to his 50-year-old son, and he isn't ready. Being the decision maker for someone else is a much harder thing to do," says Sachs, who says the role requires more education than is typically given.

Promoting this kind of understanding, however, requires communication and counseling with doctors. "Counseling takes time  and requires adequate reimbursement for the physician," says Mitchell.